Frequency modulation system



J. R. SHONNARD FREQUENCY MODULATION SYSTEM Jan. 2, 1945.

Filed March- 6. 1943' RADIO TRANSMITTER I mxsn FACSIMILEI LP F ILTER RADIO RECEIVER SIGNAL.

LEVEL a 0 m m U 3 o o VI M m E3 m6 mmn 2 mumm m e s m l R m 56 w P s mw\ I| m\ 2 3 nv 5 2 l N OR v m 224 4A ATTORNEY LIGHT INTENSITY INVE 8Y5 FIG. 2

new Jan. 2, 194 5 .f-" E S T ATENT;01 mg This invention relates to telephoto or telefacsimile systems, and more particularly to a system of this character employing a constant-ampli tude, variable-frequency sub-car ier.

messes msqvnncr MOD ATO sYs'rEM John a. Shonnard, New York, a; Y., assignor to Times Telephoto Equipment Inc., New York, N.,Y., a corporation of New York Application March 0, 194:, Serial no. 478498 5 Claims. (c1. 178-6) using a recording lamp in the receiver. I The transmitt ng and recordingdrunis may be hased as shown in thepatent to A. G. Qooley No. 2,275,- 249 dated March 3, 1942. and the drums may be Frequency modulation has has proposed for 5 \driven by constant-speed motors to-avoid the facsimile transmission but the prior systems have been rather complex and diflicult to line up and maintain in adjustment.

frequency-modulated sub-carrier present outif desired. However the use' of a hecessity' for transmitting synchronizing signals,

The output signals fromthe facsimile transmitt'er H) are rectified in the full-,wave rectifier standing advantages where high quality. record, 10 ll and smoothed out in low-pass filter l8. It

ing is desired, especially where a long radio channel is-employed, since the distortion produced by fading, echo, parasitic disturbances and the like is substantially eliminated.

Consequently the'obin light or Y may be assumed for purposes of explanation that the output signal level variations from transmitter iii are irectly proportional to the changes ading of the picture or copy being .i Of y invention. in general-terms, is to'sim I scanned. This relationship is represented by the pills systems of this character in wliich'the frequency of the sub-carrier is varied inaccordance' with signal variations.

"improve wave-signalling low-pass filter having a linear irequency-attenuationcharacteristic as a signal discriminating 5 IBM (LP. 3.. the-,fllter element at the receiver.

desired;

systems "employing a C. P. 8.; forexample.

graph of Fig. 2. If a non-linear relationship between signal and lightiplck-up' is involved, suitable correction or compensation can be made'if The low-pass filter I6 is designed to reduce the component of the rectiiled current resultingfrom the alternating-current variation or chopping impulses employed in connection with the opticalpick up' in the. transmitter ID. If the latter is lBmaycut oil? at 1200 The rectified signal variations in the output a. of the filter It are; applied ton frequency modulatertube 20 arranged to control the beat freon the accompanying drawing, wherein 30 quency generated by the'oscillators 2| and 22, in

. Fig. 1 i s. circuit diagramor a phototransmis+ sion system emb'o'dyingthe invention, and v Figs. 2 and 3 are graphs illustrating the rela-' 'tion between light intensity. signal amplitudeand c'onnection-withthe mixer23. The variable-ire-- quency signals are controlled in accordance with the invention, explained in detail hereinafter,

so that a linear relation obtains between the rrequencyin the system or Fig. 1. -.35 variations in power or the amplitude-modulated sFor purposes of illustration... the invention picture signals and the variations .in frequency .shown as emigodied in a phototransmission'sysg; the sub-carrier generated by the oscillators 'tem having atra'nsmitter and receiver of conyentional type but is not necessarily limitedyto and 22. The frequency-modulated sub-carrier is amplified in amplifier andutilized to modusuch a system. It may also be'utilized in con- 10 late the-radioi'requency carrier, as indicated.

nection. with other systems involving the trans-. m m of variable signals over a radio link-or 1 high frequency channel. Referring to Fig. lei thearawmg; the system shown comprises a'tac-..

- At the receiving station, the signals from the .radio' transmitter are detected and applied through thevlimit'er :1- diow-pass-mter 2a to annmplifler 2'0 and 'lampor' recording device 30 simfl transmitter 10 arranged to modulate a 4 otltheg iacs'imile recei'ver i2.- Aashown, the lowradio transmitter l;|. The receiving station com --prises a, radio receiver. l2 connected to a. iac-- similereceiver or recorder ll adapted to repro-.-

duce the photograph or copy on the drum of the filter 28 may comprise series inductance and capacitance. and" a simple filter "of. this typehgs a substantially linear frequency-attenuationxcharacteristic over the working range. The

transmitter; It in response to transmitted signals. BOIassu 'ned linear.char: 'acteristic or the filter '28 is The transmitter in and receiver I! are illustrated 'as or the drum type, and may be or any conven I tional construction; for example, as shown in represented-by the dotted line in Fig.3. It is also med that 711 1 811111 level variesfrom about plug 12 db.-

db. from "black" tof'white,

' the prior Patent No. 2,015,742, to A. ,G. Cooley" 7 and that the. corresponding frequency shift oi.

7 dated October 5, 1935 or of the well-known type '55 suh -carrier-is 34 00"C. S. a

C. P. S. In cases'where because of apparatus limitations or other reasons'it is desirableito empioy other ranges than those referred-ta. appropriate adjustment or changes in the amplifiers, oscillators, filters and recording equipment could be made, while still using the principle ofthe invention. a

since the frequency-attenuation characteristic of the simple low-pass'filter. may be made linear over'the entire range oi frequency shift of the sub-carrier, the output'ofthe filter consists .of amplitude variations proportional to the changes in frequency of the frequency-modulated subaseases I change in th frequency of oscillator it, asthe" level increases, unless the screen-grid resistor 38 is employed. When th resistor 88 is used,

as the tube 20 draws-more current, th potential of the screen grid becomes less positive than the plate potentialand the combined change in cathode-plate impedanc and cathode-screen grid impedance maintains the desired linear relation betweeninput power changes and the frequency changes in the sub-carrier. I described Various changes in the arrangement above iorthe purpose of explaining the invencarrier; and these amplitude-modulated signals to control the receiverwill correspond over the entire range to the variations in power at the output of the facsimile transmitter ill i'fthe frequency modulation system mat quency characteristic of the frequency modulation system of Fig. 1 which in connection with a es the lineardemodulation characteristic of the filter 28. "The curve 33, Fig. 3, represents the amplitude-freticn'may be madewithout departing from the scope. thereof as defined in the appended claims.

-' I claim: s ..1. In a. facsimile transmitter adapted to generate a frequency-modulated sub-carrier, thev combination of an oscillator, a vacuum tube amplifier of the screen-grid type connected'to the resonant circuit of said oscillator to control the frequency of itsoutput'current, means for impresslng variable-amplitude picture signals on low-pass filter 28 having a linear frequency-attenuation characteristic at the receiving station provides substantially distortionless recording without elaborate compensating circuitsor apparatus. On the other hand, the curve 34 represents the amplitude-frequency characteristic of the modulation system of Fig. 1 when a triode is substituted f-o'rthewacuum tube amplifier 20, the

the input to said amplifier and means including a'resistor connected. between the screen grid and plate ofsaid tube for changing then'ormal tube impedance curve or characteristic and therebyaiIecting the'modulation characteristics of the combination set forth.

varying amplitude picture signals, after rectiii cation. being impressed upon the control grid of said triodo. The discrepancy between the curve It and the dotted line representing the characteristic of filter 28 renders evident the reason why such an arrangement is unsatisfactory, and it is difficult to match the filter to the modulation characteristic of the modulator.

The frequency modulator embodying the invcntion comprises the vacuum tube" 20 coupled circuit 38 of oscillator 2i whereby the tubeimpedances are effectivelyconnected in the reschant circuit and variations in the grid and plate impedanccsare reflected in frequency variations in the output of the oscillator 2|. 'I'he vacuum tube amplifier for obtaining a. linear 40' through a small condenser 35 to the resonant relation between the amplitude of the input sig- V nals and the frequency oi the oscillator output current.-

frequencies of the oscillators Hand 22 may be for example approximately 100 to 150. kl iocycles 1 or more, and the. beat frequency offthe sub-i between, say,'1800 C. P. B." and;

carrier may vary 34.00 C. P. B.

The tube to is a'high we pentode' amplifier,

for'example type 7C7; but.mayb any similar tetrode or pentode, preferably-a tubein. which screen-cathode capacity. suppressor-cathode ca- .p acity,and plate-cathode capacityarelow, In-the case ofa'pentoderthe s ppressor is connected to plate. as shown. The input circuit'may -in-Ei f w ,ciude a' gridbias resistor- 81. The'screengridpf tube" isconnected to -:-the' plate of the tube through a resistor." of, say, one-half megohm-,.-

-: and the plate is-conne'ctedi-to the high-voltage T source through aresistor 8| onlsay, one quarter unegohmr 1 The describe similar to conventional systems with the exception-ofthe resistive ii -plate. connection. It baa-been found that this connection results .in a. vconstanturoportionality between the vari- ,ations in frequency: of sub-carrierland' the.

'3 arrangement is substantially:

- 3. Ina facsimile transmitter adapted to gencrate a frequency-modulated sub-carrier, the

combination of an oscillator, a vacuum tube amplifier of the screen' grid typ having its plate impedance effectively in the resonant circuit of saldoscillator to control thefrequency' ofits output current. means-.ior impressing signals, of

varying power. on theinputto s'aid vacuum tube. amplifier. a' resistor of'the order of V: megohm connected between the screen-grid and plate of v said vacuumtube amplifier and means for-"supjplying positivepotential. to the-plate through a second resistor ofthe order oi megohm.

'4. A facsimile system comprising sending and receiving stations, a vacuum tube amplifier-"of the screen g'rid type'at the sending station, means inclddihg'an oscillator for generating afrequency inodulated sub-carrier at said. send-ins station.means,for impressing picture signals of arying power on thein'put to said amplifier.) Qla small-condenser coupling the plate circuit'oi" said amplitierato s'aid oscillator to control the frequency of its output'current in accordance with 'saidw-signals of varying power, a resistor connected between thescreen grid and plate of said amplifier for obtaining a linear relation bevariations inthe powor of the input sign is. shown by actual tests (see curve 83. Fig. '3).

The gridbias on the modulator control tube varies logarithmically with the power of the. in-

-- put signal. and this causes a'disproportidnate tween the power of the input signals'and' the frequency of the sub-carrier, and receiving means ,at the receiving station for reproducing transmitted" facsimile. copies in accordance with-the frequency variationsofsaid sub-carrier, said re ceiving means including a limiter, slow-pass fiiter having a lubltantially-linear frequency-ate tenuation characteristic and recording means re- 7 sponsive to the modulated signal output variations from said filter.

5. A frequency modulation facsimile system in which picture signals of varying amplitude are converted into frequency variations of a subcarrier characterized in that a screen grid tube 1 is connected to be responsive to the signals of varying amplitude and to control the .sub-car- 

